Temozolomide is a second generation imidazotetrazine and is the methyl analog of mitozolomide, the first generation imidazotetrazine. Although mitozolomide had shown promising results in animal models, hematologic toxicity in human trials was unpredictable and sometimes severe, precluding further clinical trials. The specific aims of this study are: (1) to determine the maximum tolerable dose (MTD) of oral temozolomide given daily for 5 days; (2) to determine the toxicities of temozolmide given on this schedule; (3) to determine the pharmacokinetic behavior of temolzolomide administered orally for 5 consecutive days in pediatric and adolescent patients; (4) to determine the antitumor activity of temozolomide within the confines of a phase I study.